The present invention relates to a pattern forming method in which a desired pattern is formed utilizing a lift-off method.
In general, a lift-off method uses resist made of organic material which is soluble in organic solvent.
A conventional lift-off patterning method will be described with reference to FIG. 1. After resist 2 has been applied to one surface of a substrate as shown in FIG. 1a, a resist pattern is formed by photo-etching. It is desirable that the resist pattern thus formed be such that the resist in cross-section has a greatly inverted taper as shown in FIG. 1b. Then, film layers 3 are formed on the surfaces of the resist layer and on the exposed substrate 1 by vacuum evaporation or sputtering the material in a desired pattern as shown in FIG. 1c. The substrate 1 with the resist layer and the film layers 3 is immersed in a solvent. As a result, the resist 2 is dissolved in the solvent and therefore the film layer 3 on the resist layer 2 is removed. Thus, only the film layer 3 on the surface of the exposed substrate 1 remains to provide the desired pattern.
With this method, it is necessary to form the resist layer 2 relatively thick so that the film layer 3 on the surface of the exposed substrate 1 does not touch the film layer 3 on the resist layer 2. Accordingly, it is difficult to etch the resist layer 2 with a high accuracy and therefore the pattern formed by the film layer 3 is low in accuracy. This is one of the drawbacks accompanying the above-described conventional method.
A conventional method of providing a finer pattern than that formed by the method of FIG. 1 is illustrated in FIG. 2.
According to the second conventional method, a first resist layer 4 and a second resist layer 2 are successively formed on a substrate. Then, a first pattern is formed by photoetching the second resist layer 2. The pattern thus formed must have a considerably high dimensional accuracy and it must be opposite to or complementary to the pattern of a film layer 3 which is formed later.
Thereafter, a second pattern is formed by photoetching the first resist layer 4 or the like with the pattern of the second resist layer 2 as a mask. In this case, the pattern of the first layer 4 is developed more than the pattern of the second resist layer 2 so as to provide a resist section having overhanging portions as shown in FIG. 2b.
In the following step, a desired pattern is obtained by treating the aforementioned film layers 3 formed on the surfaces of the second resist layer 2 and of the exposed substrate 1. In this step, the above-described structure contributes greatly to the lifting off of the film layer 3 formed on the second resist layer 2 by vacuum-evaporation or the like. That is, in the lifting-off operation, a solvent dissolves the resist layers beginning with the lower resist layer 4 and therefore the lifting-off operation is readily accomplished to thereby obtain the desired pattern.
In accordance with this method, the resist layer 4 is interposed between the resist layer 2 and the substrate so that it is thus possible to make the resist layer 2 quite thin. Accordingly, the method of FIG. 2 can provide a finer pattern than the method of FIG. 1. However, the second method is still disadvantageous in that it is impossible to completely avoid the etching of the resist layer 2 while the resist layer 4 is being etched and hence the finally formed film layer 3 has a relatively low dimensional accuracy.
In addition to the above-described conventional methods, methods of using a polyamide material and an oxide film, respectively, instead of the resist layer 4 are known in the art. However, the first of these methods is disadvantageous in that it does not provide a sufficient resolution to form a fine pattern. The latter method is disadvantageous in that during etching of the oxide film, the undercut control is rather difficult if chemical etching is employed and, if plasma etching is employed for etching the oxide film, then the resist layer 2 is also etched and therefore it is difficult to obtain a pattern with the desired dimensional accuracy because the oxide film etching speed is not very high.
The above-described method in which the oxide film is interposed between the resist layer 2 and the substrate 1 is further disadvantageous in the following point. In the case where an oxide film has been previously formed on the substrate for a different purpose, it is impossible to etch only the oxide film in the middle step. Accordingly, the method cannot be applied to a material in which at least an oxide film has previously been formed on the substrate.
For the methods in which the resist layer 4, the polyamide or the oxide film is interposed between the resist layer 2 and the substrate 1, it is difficult to satisfactorily control the dimensions of the overhanging portions with a high accuracy. Especially, in the method employing the resist layer 4, the resist is considerably thick and therefore it is extremely difficult to produce a very fine pattern in which the dimensions must be controlled in the sub-micron range.